A torque converter for an automobile includes a pump impeller connected to a crankshaft of an engine, a turbine runner connected to a main shaft of a transmission, and a stator supported on a casing via a one-way clutch, drives the turbine runner by means of a flow of oil generated by the pump impeller, and aligns, by means of the stator, the flow of oil that has passed through the turbine runner and been deflected, thus circulating oil along the pump impeller, the turbine runner, and the stator.
FIG. 14 (A) shows a row of blades of a stator 01 of a conventional high torque ratio type torque converter, and a stator blade 02 thereof has a large camber in order to achieve a high torque ratio. With regard to such a torque converter, in a region where the speed ratio (output rotational speed/input rotational speed) is small, oil that has passed through the turbine runner flows into the stator blade 02 from the ventral surface side and flows smoothly (arrow a), but in a region where the speed ratio is large, since oil that has passed through the turbine runner flows into the stator blade 02 from the dorsal surface side (arrow b), there is oil flow separation on the ventral surface side of the stator blade 02 where the camber is strong, and there is the problem that the torque ratio (output torque/input torque) becomes less than 1 or the capacity coefficient (input torque/input rotational speed) decreases greatly.
Conventionally, in such a case a one-way clutch supporting the stator 01 on the casing is disengaged, and the stator 01 is made to idle in the direction of arrow c, thus suppressing any decrease in the speed of the stator blade 02 and thereby improving performance.
FIG. 15 shows changes, in response to speed ratio, of the torque ratio and the capacity coefficient of a high torque ratio type torque converter (solid line) and a high capacity type torque converter (broken line). The high torque ratio type torque converter has an excellent torque ratio in a low speed ratio region compared with the high capacity type torque converter, but there is the problem that the capacity coefficient in a high speed ratio region is still low.
In order to further improve the capacity coefficient in the high speed ratio region, as shown in FIG. 14 (B), an arrangement in which a stator blade 02 of a stator 01 is divided into a first stator blade 02a on the upstream side in the direction of flow of oil and a second stator blade 02b on the downstream side in the direction of flow of oil, and the first stator blade 02a and the second stator blade 02b are supported on a casing by respective one-way clutches so that they can idle independently from each other is known from Patent Document 1 below.
Due to the stator blade 02 being divided into the first stator blade 02a and the second stator blade 02b in this way, the first stator blade 02a is first made to idle in response to an increase in speed ratio, thus allowing the function of the second stator blade 02b in aligning the flow of oil to be exhibited, and in response to a further increase in the speed ratio both the first stator blade 02a and the second stator blade 02b are made to idle, thus preventing oil flow separation.